Mecha-Ude: Mechanical Arms

Hikaru Amatsuga is a middle school student who lives in Kita-Kagami City and one day, he hears cries for help and finds a mechanical lifeform in the shape of an arm named Alma. Before he knew it, Alma ended up connecting (derizing) to his hoodie.

The Mecha-Ude user Aki Murasame transfers into Hikaru's class with the intention to secure Alma. She belongs to an organization named ARMS and she starts following Hikaru and Alma around 24/7 and Hikaru seems completely exhausted because of all of this.

Hikaru and Alma are taken to ARMS HQ. There, Hikaru is told that he is not suited to be a Mecha-Ude user, and Alma falls under ARMS' protection. Meanwhile, multiple incidents where Mecha-Udes are forcibly ripped away from their users occur.

Hikaru was dealing with the fact that he had to still go to school while learning to live with Alma, working as a member of ARMS, and being followed around by Aki.

Aki is finally able to track down the worm-type Mecha-Ude user who killed her family. But the person she finds is her younger twin sister Fubuki, who should've also died in the incident that killed Aki's family. Aki is told the truth about what happened in that incident during her unexpected reunion with Fubuki and she's informed of one other cruel truth.

An incident where most of the Mecha-Ude users in Kita-Kagami City lost their minds and go berserk occurs. Hikaru tries to find out more about what the cause of this is, which is the worm-type Mecha-Ude, Amaryllis. But he's also trying to find out why Aki seems to be working with Amaryllis and if she actually has any part in this incident.